1. Field of the Invention
The present invention relates to an antenna and a non-contact tag, and more particularly to an RFID (Radio Frequency Identification) antenna and an RFID non-contact tag, for performing transmission and reception with an RFID reader/writer.
2. Description of the Related Art
An RFID system in which non-contact tags (hereinafter referred to as “the RFID tags”) having identification information embedded therein are attached to respective articles or persons, thereby enabling transmission/reception of information between the persons or the articles and RFID reader/writers (hereinafter simply referred to as “the reader/writers”) using a radio signal, is expected to be applied to various fields, such as management of factory production, management of physical distribution, and management of room entrance/exit, and is coming into practical use.
The method of communication therefore is classified into an electromagnetic induction method and a radio-frequency method. The electromagnetic induction method mainly uses electromagnetic waves of 135 kHz or 13.56 MHz, and transmits/receives information by induced voltage caused between the antenna of an RFID tag and the antenna coil of an RFID reader/writer. The communication distance is limited to the maximum of approximately 1 m.
On the other hand, the radio-frequency method uses radio waves in a UHF band (860 to 960 MHz) or radio waves of 2.45 GHz, and performs communication between the antenna of an RFID tag and the antenna of an RFID reader/writer. However, since the radio waves of 2.45 GHz are short in wavelength, communication trouble can occur due to obstruction. Therefore, recently, attention is being given to RFID systems using radio waves in the UHF band.
In the following, a description will be given of a RFID system using a radio signal in the UHF band.
In communication between an RFID tag and a reader/writer, first, the reader/writer sends a signal of approximately 1 W to the RFID tag, using a radio signal in the UHF band, and the RFID tag receives the signal and sends a response signal back to the reader/writer again. Through the signal exchange, the reader/writer can read information stored in the RFID tag. The communication distance depends on the gain of a tag antenna, the operating voltage of an IC (Integrated Circuit) chip, and the ambient environment, but is within approximately 3 m.
The RFID tag comprises the antenna and the IC chip connected to the antenna. The IC chip has a size of several mm or less, but the antenna basically requires a length of a half wavelength λ/2. For this reason, when the UHF band is utilized, the antenna requires a size of approximately 150 mm, and the performance of the RFID tag largely depends on the size of its antenna.
FIG. 14 shows an equivalent circuit of the RFID tag.
The IC chip can be equivalently expressed by parallel connection of a resistance R1 and a capacitance C1 (e.g. 1 pF). As the resistance R1 of the IC chip, a large resistance of approximately 1000 Ω is used so as to maintain a driving voltage of several volts with respect to a predetermined electric power. On the other hand, the antenna can be equivalently expressed by a parallel connection of a radiation resistance R2 (e.g. 1000 Ω) and an inductance L1 (e.g. 28 nH). By connecting the IC chip and the antenna in parallel, and performing impedance matching between the two, resonance occurs between the capacitance C1 and the inductance L1 and the imaginary number component is reduced to substantially zero to achieve the impedance matching, whereby electric power received by the antenna is sufficiently supplied to the IC chip.
By the way, the radiation resistance of a single dipole antenna is approximately 72 Ω, and hence it is required to increase the radiation resistance so as to achieve impedance matching with an IC chip of the above-mentioned type.
Conventionally, there has been proposed a folded antenna described below.
FIG. 15 is a view of the arrangement of the folded antenna.
The illustrated conventional folded antenna 80 comprises two dipole antennas 81a and 81b each having a length of approximately 150 mm and arranged close to each other in parallel, with a spacing of e.g. 10 mm therebetween. The dipole antenna 81a and the dipole antenna 81b are connected to each other at opposite ends thereof, and electric power is fed via a feeder part 82 formed in the center of the dipole antenna 81a. With this arrangement, the radiation resistance R2 appearing in FIG. 14 can be made more than four times larger than the radiation resistance (72 Ω) of a single dipole antenna. Further, by changing the ratio of the line width of the dipole antenna 81b to that of the dipole antenna 81a as shown in FIG. 15, it is possible to adjust the radiation resistance such that it is increased to approximately 1000 Ω (see e.g. “Antenna Technology Handbook” (Ohmsha Ltd.), the Institute of Electronics, Information and Communication Engineers (IEICE), October, 1980, pp. 112-115).
However, although it is desirable for practical use that an RFID tag has a size not larger than a card size (86 mm×54 mm), for example, the conventional folded antenna requires a length of a longitudinal side thereof of approximately 150 mm for receiving a radio signal in the UHF band. The length is too large for practical use.